1. Field of the Invention
The present invention relates to a plastic lens such as an fθ lens molded with a molding mold member, using plastic as a material, and a molding mold therefor.
In particular, the present invention relates to a technology of preventing optical characteristics of an fθ lens used in a scanning optical system of an image recording apparatus such as a color laser printer or a color copier from being impaired in molding with a mold.
2. Description of the Related Art
Conventionally, in a scanning optical apparatus used in a laser printer or a digital copier, an imaging optical system having fθ characteristics is used as a scanning optical system when a light beam from a light source is scanned by a light deflector and focused in a spot shape on an image-forming surface on a photosensitive drum.
The fθ lens needs to have a shape portion as a reference for positioning in a longitudinal direction with respect to a scanning optical apparatus body.
Therefore, in an fθ lens 801 having a gate portion 803 as illustrated in FIG. 6A, a convex portion 802 as a shape portion of a positioning reference is formed in a direction parallel to a direction of an optical axis 806 and meshed with to a reference receiving portion 804 of a scanning optical apparatus 809 as illustrated in FIG. 6B.
Further, as illustrated in FIG. 6C, in order to prevent a parting line from being complicated and suppressing a mold production cost, there is known an fθ lens in which a convex portion 805 of the positioning reference is provided in a direction orthogonal to the optical axis 806 (see Japanese Patent Application Laid-Open No. H06-160747).
In such an fθ lens, as illustrated in FIG. 6C, the convex portion 805 as a positioning reference is provided in a longitudinal side surface portion of the lens, and the convex portion 805 is fitted in the reference receiving portion 804 of the scanning optical apparatus 80 as illustrated in FIG. 6D, and thus, positioning of the fθ lens in the longitudinal direction is performed.
It is preferred that positioning in the longitudinal direction in the fθ lens be performed in the vicinity of the lens optical axis 806 for adjusting the optical characteristics.
However, when only one convex portion as a positioning reference is formed on the longitudinal side surface portion of the lens as described above, the convex portions as a reference may interfere with each other between lenses during the storage of produced lenses and during accommodation of jigs used for attachment to the scanning optical apparatus.
Therefore, a twist is caused in the jig, which is not preferred for smooth attachment to the scanning optical apparatus. As a configuration solving this problem, the arrangement in which the convex portions 805 as a positioning reference are placed in a staggered manner as illustrated in FIG. 6E has been developed.
Further, in order to prevent the interference between the lenses, the arrangement in which the convex portion is replaced by a concave portion has been developed.
The positioning reference of the convex portion or the concave portion placed on the side surface of the fθ lens is shaped by side surface forming slide blocks.
At this time, the fθ lens generally has a thick and deflected shape in terms of a function.
Therefore, a high molding pressure is required for molding so that sink is not formed in the thick portion.
As a result, the positioning reference portion in a convex or concave shape as a small and fine shape described above is supplied with a pressure more than necessary for forming, and the engagement force with the slide blocks for forming the positioning reference shape increases.
Therefore, the fθ lens molded at a time of opening of the slide blocks during releasing has its positioning reference shape deflected in the side surface direction with the gate to be fixed by a runner and a spool being a fulcrum, which causes mold releasing from a cavity forming portion on a movable side.
Consequently, formation defects such as abrasion involved in mold releasing from the cavity forming portion of a movable side mold and fracture of the positioning reference itself are caused.
Conventionally, Japanese Patent No. 3814591 proposes that the above-mentioned releasing defect is avoided with respect to the arrangement in which the convex portion of the positioning reference is formed in parallel with the optical axis direction as illustrated in FIG. 6A.
In Japanese Patent No. 3814591, a shape portion for restricting the expansion during releasing is placed at an end portion on a side opposite to the gate in the longitudinal direction of the fθ lens as countermeasures.
More specifically, the fθ lens that is long in one direction expands in the longitudinal direction during releasing. Therefore, when the convex portion as a positioning reference is released from a mold cavity portion, a large force is applied to the convex portion, and an eclipse is caused in the convex portion due to the releasing defects.
As illustrated in FIG. 6F, Japanese Patent No. 3814591 adopts a configuration in which, as a shape portion for restricting the expansion during releasing, a convex portion 811 protruding in a direction parallel to the convex portion 802 as a positioning reference or a concave portion 812 receding in the direction is formed in an end portion on a side opposite to the gate in the longitudinal direction of the fθ lens.
Due to this configuration, the force expanding in the longitudinal direction is received, and an eclipse is avoided.
However, the above-mentioned conventional examples have the following problems.
In Japanese Patent No. 3814591, the deflection in the lens side surface direction caused by the engagement with the slide blocks forming the positioning reference cannot be avoided.
Therefore, it is difficult to avoid the problems caused by releasing defects such as an eclipse, peeling, and tearing with respect to the positioning reference caused by the deflection.
Further, in the arrangement in which a convex or concave positioning reference portion is placed on an upper lens side surface, which is different from the arrangement in which a convex portion of a positioning reference is formed in parallel with the optical axis direction as in Japanese Patent No. 3814591, the following problems newly arise.
The factors of the deflection in the fθ lens side surface direction are described with reference to FIGS. 7A to 7E.
Before describing the deflection, the outline for the molding with a mold for such an fθ lens is described with reference to FIG. 7A.
As illustrated in FIG. 7A, a resin injected from a plasticizing apparatus 910 flows through a spool 909, a runner 908, and a gate 907 to fill a lens cavity 901, and a cavity shape is transferred with a molding force added from the plasticizing apparatus.
FIG. 7B is a cross-sectional view taken along the line 7B-7B of FIG. 7A during releasing. FIG. 7C is an enlarged view of a C portion of FIG. 7B, and FIG. 7D is a cross-sectional view taken along the line 7D-7D of FIG. 7B.
A convex portion 1002 as a positioning reference placed on a lens side surface is formed by slide blocks 1004 as illustrated in FIG. 7B. A thick molding such as an fθ lens requires a high dwell force so that sink is not formed in the thick portion. As a result, in molding both a thick portion 1001 and a small shape portion of the convex portion 1002 in the longitudinal direction of the fθ lens, an excess pressure is applied to the convex portion 1002 that is a small shape portion, and an engagement force 1011 with respect to a mold portion for molding the convex portion 1002 becomes high.
For releasing of the small shape, the force overcoming the engagement force is required.
However, in molding of a rectangular lens that is long in one direction with one end in the longitudinal direction being a gate 1012, such as an fθ lens, as illustrated in FIG. 7B, the lens expands in a longitudinal direction 1010 with the gate 1012 being a binding point during releasing.
Therefore, a contact strength 1007 with respect to a mold member 1006 constituting the movable side cavity is reduced largely (FIG. 7D).
Consequently, large deflection occurs in the side surface direction of the lens due to a sliding 1008 of the side surface slide blocks caused by the keeping of the engagement force 1011.
Due to such deflection, in the case where the concave portions of positioning reference portions are placed in a staggered manner on the lens side surface, the positioning reference 1002 on a side far from the gate 1012 is deflected in a certain direction.
The reason for this is considered as follows: in the case of setting the gate side of a mold used for molding as a fixed point, a force larger than a moment force acts.
In order to restrict the above-mentioned expansion in the longitudinal direction of the lens during releasing, it is most effective to bind the end portion in the longitudinal direction of the lens on a side opposite to the gate side by a mold member 1014 that does not slide, as illustrated in FIG. 7E.
However, when the expansion in the longitudinal direction is restricting, a stress 1015 remains in the fθ lens, and a refractive index becomes non-uniform to influence the optical characteristics.
Therefore, in order to obtain satisfactory optical characteristics, it is necessary to release a residual stress remaining in a molding due to the expansion in the longitudinal direction.
Thus, when a lens that is long in one direction, such as the fθ lens, is molded with a mold, it is necessary to prevent the refractive index from being non-uniform due to the residual internal stress, without impairing a convex or concave shape of a positioning reference placed on the side surface of the lens during releasing.